Synthetic elastomers, in general, are used in a variety of applications, either as such, or as modifiers of the mechanical and/or rheological properties of other polymers or materials. However, many elastomers, such as styrene-butadiene rubbers (SBR) in raw form, tend to agglomerate and form blocks, which in part gives them their usefulness in a variety of applications such as adhesives or impact modifiers, but that makes handling of the same in raw form very difficult.
For instance, the most common raw elastomers of commercial type are SBR elastomers, with styrene monomer content fluctuating from 3 to 45% w/w, with the balance being butadiene. By having the most content of butadiene, one of the properties that is immediately apparent in these materials is their stickiness, which is mainly attributed to the glass transition temperature (transition from solid to viscoelastic stage) that is less than −20° C. In particular, this stickiness is increased when the polymers are heated to temperatures above ambient temperature, and even more if pressure is applied. This phenomenon of stickiness is increased when butadiene and styrene are in a mainly random form and blocks of polystyrene are reduced.
This tackiness property makes impossible to commercialize this type of SBR elastomers in the form of free-flowing pellets, crumbs or particles, inasmuch as over time, the adherence between the particles is increased thus forming naturally blocks, phenomenon that is also known as “cold flow”. Because of this fact, regardless of the polymerization process, such SBR elastomers are sold in blocks, usually weighing more than 25 kilograms.
For this reason, raw elastomers are usually commercialized in the form of big blocks and need to be further cut, milled or somehow processed to reduce them to smaller particles depending on the final application. Such processing to reduce particle size must be done on-site or at a time very close to final processing because otherwise the raw elastomer agglomerates again and form solid big blocks.
In order to maintain temporarily raw elastomers in small particles, different anti-block or partitioning agents are used in the prior art, ranging from talc or inorganic fillers to complex compounds with resins, other polymers and waxes have been used. However, the majority of these agents have at least one of the following disadvantages:
a) They require some kind of treatment (heating, chemical or of some other kind) in order to be applied or after applying it to the elastomer.
b) They jeopardize the properties of the elastomer in the final application by compromising tackiness, mechanical or flow properties.
c) The amount required for anti-blocking activity is very high.
d) The anti-blocking effect is limited in terms of temperature conditions and length of time.
Of course all of the above disadvantages impose serious applicability or economical burdens that make the majority of the anti-blocking agents unfeasible or limited in use to elastomeric compositions that have been further formulated, cross-linked or vulcanized, so that with less amount of agent the desired anti-blocking effect is achieved at least temporarily. However, when applied to virgin or raw elastomeric compositions, such agents are usually not useful in the long term so as to be able to commercialize, store and process the same in particle, crumb or pellet form.
One of the applications that are more sensitive to the above described problems is asphalt modification through elastomeric compositions.
Asphalt is a viscous, sticky and black material used as a binder in pavement mixtures for the construction of road surfaces, as well as in waterproofing. In view of the fact that asphalt is a highly impermeable, adhesive and cohesive material, able to withstand high stresses instantaneous and flow under the action of constant loads, it has ideal properties for the construction of road surfaces complying with the following functions: i) waterproof the pavement structure, making it very sensitive to moisture and effective against the penetration of water from rainfall; and, ii) providing an intimate union and cohesion between aggregates, capable of withstanding the mechanical action of disintegration caused by the loads of vehicles.
However, in its virgin state asphalt has rheological and thermomechanical properties limiting its application and usefulness life in pavements. For instance, the softening typical temperatures of virgin asphalts are in the range of 50° C. to 60° C. (rheological-failure temperatures or RFT). However, such temperatures can be improved with the use of polymers that allow RFT higher than 60° C., thus making asphalts more resistant to friction loss associated with rolling of vehicles.
Another interesting property is that virgin asphalts have performance degrees of the PG type of SUPERPAVE (Superior Performing Asphalt Pavements) typical for PG 64-10. The PG range can be extended to 82-22 PG grades with the help of the polymers.
Asphalts modified with polymers were developed with the intention to reduce the requirements of energy for the production, storage and application of asphalts cement in roads. The modified asphalts use polymers that can form three-dimensional networks through vulcanization, formation of crystals or entanglements of the polymer chains.
The objects pursued with the modification of asphalts with polymers are:
a) Having more viscous binders at elevated temperatures in order to reduce the permanent deformation of mixtures composing the bearing layers and increasing the stiffness;
b) Reducing cracking due to thermal effect at low temperatures and fatigue, thus increasing its elasticity; and
c) Having a binder with best adhesive features.
SBR elastomeric compositions with molecular weight (Mw) in the range from about 50,000 to 500,000 Daltons in raw form are mostly used to modify asphalts. As mentioned before, these elastomers are commercialized and stored as blocks which subsequently have to be crushed or grinded by the users on-site, thus increasing the costs in the total process of asphalt modification due to both, energy consumption, labor and time.
There have been various solutions tried in the prior art in order to solve the aforesaid problem.
One solution has been emulsifying SBR elastomers in water in order to facilitate incorporation into asphalt. However, for polymers with more than 100,000 Daltons of Mw great difficulties for emulsification have been encountered, and in addition, the high temperatures used in the modification processes of the asphalt, often higher than the evaporation temperature of water, origins flashing or sudden streams of water that can cause accidents, in addition to the oxidation of the asphalt that implies a premature aging factor, which reduces its useful life time in the application of pavement.
These emulsified elastomers have found some application in asphalt emulsions, also well known in the state of the art. These emulsions are mixtures of asphalt with emulsifiers that form a stable emulsification with water, which allows its application on cold asphalt surfaces, that is to say, at temperatures lower than 100° C.
One of these technologies is found in U.S. Pat. No. 6,136,899 wherein a specific type of SBR emulsion can be used to modify asphalt cement to greatly enhance the resistance to shoving, rutting and low temperature cracking of asphalt concretes made therewith. Such SBR emulsion is compatible with virtually all types of asphalt and the modified asphalts have extremely high levels of force ductility, tenacity and toughness. According to this patent, SBR used to modify asphalt cement in the practice of this invention is a blend of (i) a high molecular weight (Mw) styrene-butadiene rubber having an average Mw of at least about 300,000 and (ii) a low Mw styrene-butadiene rubber having an average Mw of less than about 280,000; wherein the ratio of the high Mw styrene-butadiene rubber to the low Mw styrene-butadiene rubber is within the range of about 80:20 to about 25:75, and require further features in the elastomer composition in order to work.
On the other hand, in the U.S. Pat. No. 6,503,968 an asphalt modifier of styrene-butadiene-styrene block copolymer and styrene-butadiene latex is disclosed, which relates to an asphalt modifier comprising 5-30 wt % of dispersed styrene-butadiene-styrene block copolymer and 70-95 wt % of dispersed styrene-butadiene latex, which has an excellent resistance property against plastic deformation, while preventing the occurrence of crack at a low temperature. However, this technology, as mentioned before, has the disadvantage of the need of evaporating water in order to recover the elastomer and in some cases, particularly when modifying the asphalt in temperatures higher than 100° C., with the problems described above. In addition, its application require excess of polymeric latex emulsion, considering that the solids content of these materials fluctuate at levels less than 70% solids, which impacts the final cost of operation.
An improved method for treating crumb rubber particles for use in asphalt compositions is disclosed in U.S. Pat. No. 5,927,620 which is characterized by activating the particles to enhance rheological properties thereof. A slurry of crumb rubber particles is formed by adding water thereto. The slurry is heated to a temperature of 85-90° C. to release excess oils and chemicals from the particles into the slurry. The slurry is dried to produce a fine mesh rubber product with enhanced rheological properties. Such materials have the disadvantage that it is no longer a virgin material, but a cross-linked material mixed with some loads such as carbon black which is used as the main ingredient in tire formulations.
Likewise, U.S. Pat. No. 6,884,831 relates to modified asphalts with partitioning agent and method for making the same, which is characterized by the addition of a partitioning agent to a polymer modifier material such as synthetic rubber to prevent re-agglomeration of the rubber. The partitioning agent is complex and includes a mixture of phenyl-formaldehyde resins and waxes. Whereas reduction of time required to disperse the modifier material in asphalt and lower viscosity of the modified asphalt are reported through the use of such partitioning agent, this technology requires high temperature to incorporate the partitioning agent into the polymer and, due to the resins and waxes contained therein, its use in emulsions and the incorporation in the normal process of production of raw polymers is expensive and difficult because the additive is incompatible with water and, evidently, very difficult to emulsify, thus making the whole process more expensive due to both, energy requirements for incorporation into the polymer and further treatments and additives required at the application in asphalt.
The same disadvantages are found in U.S. Pat. Nos. 7,371,794 and 7,847,006, which refers to the same kind of partitioning agent with phenyl formaldehyde resin and waxes, in the case of the latter with the addition of precipitated silica. As it is easily seen, the disadvantages of these additives with waxes and resins prevail, where the preferred temperatures for addition of the partitioning agent to the polymer are well above 100° C. and a preheating step is necessary in order to make more fluid the resin and waxes.
Likewise, the US Patent Application Serial No. 2010/0187718 relates to a method for making a modified polymer, which is characterized by re-capsulating the polymer via extrusion, through the use of a cross-linking agent that is mixed with particles of synthetic rubber material to form a mixture which is then heated and delivered to an extruding machine. The extruding machine further heats the mixture and produces strands of modified polymer material which are cooled and pelletized to form small pellets of re-capsulated modified polymer. A partitioning or anti-blocking agent of the same type than described in the other patents, namely with phenyl formaldehyde resin and waxes, is added to the pellets to prevent re-agglomeration of the modified polymer.
The modifier materials or anti-block additives described in the U.S. patents mentioned above describe properties that could solve the problem of agglomeration of raw elastomer particles, but have a lot of problems in the process due to the resins and waxes contained therein, which make it difficult and expensive to apply due to the use of temperature, and problems in the drying systems during the processes of production of the elastomers, through increasing the internal pressure in the same and with the need of further additives or processing.
Accordingly, the prior art has not suggested or described raw elastomeric compositions in free-flowing pellet form that, in addition to allowing the commercialization, storage and application as particles, enhance the properties of the rubber when used in further applications, maintaining the properties of the elastomeric composition when used as modifier for asphalts, typical rubber applications or adhesives, in spite of the use of an anti-blocking or partitioning agent. Furthermore, there are not any technologies allowing the incorporation of an anti-blocking or partitioning agent without the need of further processing or the use of more additives, without compromising the properties of the elastomer in the final applications and with anti-blocking effects achieved for long storage times and high weights, thus allowing commercialization in crumb, pellet or particle form.